1. Field of the Invention
The present invention relates to a fluorescent water base ink for ink-jet recording having a high fluorescence intensity.
2. Description of the Related Art
In the ink-jet recording system, the ink is discharged, for example, from the nozzles, the slits, or the porous film to perform the recording, for example, on the paper, the cloth, or the film. The system for discharging the ink includes, for example, the electrostatic attraction system in which the ink is discharged by utilizing the electrostatic attracting force, the drop-on-demand system in which the mechanical vibration or the displacement is applied to the ink by using the piezoelectric element, and the thermal ink-jet system in which bubbles are generated by heating the ink to utilize the pressure generated thereby. Ink droplets are formed by a variety of ink discharge systems to adhere a part or all of the ink droplets onto the recording objective material, and thus the recording is performed.
The development is advanced for the ink which is applicable in various fields by utilizing the ink-jet recording system as described above. In particular, the fluorescent ink has such a property that the light (exciting light) having a specified wavelength is absorbed, and the light (fluorescence) having a wavelength longer than that of the exciting light is emitted. Therefore, for example, the fluorescent ink is tried to be applied to such a way of use that any information, which requires the security, is recorded on a recording medium, and the information can be read by emitting the fluorescence by radiating the ultraviolet light as the exciting light. The fluorescent ink includes not only transparent inks which have no absorption in the visible region but also color inks which have the absorption in the visible region. In the case of the color fluorescent ink which has the absorption in the visible region, the information can be recognized visually even with the naked eye. The reading means can be selected depending on the contents of the information. Therefore, it is expected to widen the versatility of the ink.
However, in general, it is known for the fluorescent ink that the concentration quenching phenomenon occurs such that the fluorescence intensity is all the more decreased if the concentration of the fluorescent dye contained in the ink exceeds a certain amount, for the following reason. That is, the fluorescent dye undergoes the transition from the excited state to the more stable state without radiating the absorbed energy as the light due to the interaction of the excited fluorescent dye molecules (nonradiating process). Therefore, there has been such a problem that any fluorescent ink, which has a fluorescence intensity of not less than a certain degree, is not obtained even when the concentration of the fluorescent dye is adjusted at the best.
On the other hand, various trials have been made in order to enhance the fluorescence within a concentration range in which the concentration quenching phenomenon is not caused. For example, Japanese Patent Application Laid-open No. 2003-192962 discloses a fluorescent ink which uses, in combination, two types of fluorescent dyes which have a common absorption wavelength region in the ultraviolet region and which have a common fluorescence emission wavelength region in the visible region. When the two types of the fluorescent dyes are combined as described above, then the fluorescence intensity of the second fluorescent dye is added to the fluorescence intensity of the first fluorescent dye, and the ink having the high fluorescence intensity is obtained. However, actually, the fluorescence intensity of the fluorescent ink is smaller than the sum of the fluorescence intensities to be obtained when the two types of the fluorescent dyes are used singly respectively, probably for the following reason. That is, the first fluorescent dye and the second fluorescent dye have the common absorption wavelength region in the ultraviolet region. Therefore, it is considered that the excitation energy is shared by the first fluorescent dye and the second fluorescent dye, and each of the fluorescent dyes cannot emit the fluorescence sufficiently.